Let the height of the tree be x, the distance from the top of the tree to point A be y, and set up equations.

20+ 10=y+x- 10

(Two monkeys move the same distance)

20^2+x^2=y^2

(Pythagorean theorem)

X= 15，y=25。

You find it difficult, and so do your classmates, so it doesn't matter. Everyone is about the same level, and you are no worse than others. There are always easy questions and difficult questions in the exam, not all of them are difficult questions. Don't be afraid.

Even if you do the problem longer than others, it means that you can do it, but not as fast and skilled as others. Usually you can do two more questions than others to make yourself proficient.

Being able to understand in class means that you have a good foundation. As long as you do more questions and think more, your grades will improve. If you don't understand, you can ask the teacher, discuss with your classmates, or bring it with you. There will be enthusiastic people to help you explain.